1. Field of the Invention
The present invention relates to an autorefractometry system suitable for prescribing eyeglass or contact lenses and, more particularly, to an evoked potential autorefractometry system which uses a steady state evoked potential and which is low in cost, does not intimidate patients and provides a highly accurate refractive lens prescription.
2. Description of the Related Art
FIG. 1 illustrates a refractometer typically found in an eye doctor's office. In such a refractometer, lenses of differing refractive power are placed in front of the patient's eyes as the patient indicates whether or not the visual target has improved in focus. Such a system for prescribing eye glass lenses is subjective, requires verbal cooperation from the subject, is not suitable for communication handicapped persons and is very time consuming.
Fig. 1B illustrates a prior art acuity testing system. This movable lens system for generating steady state evoked potential amplitudes has been proposed by Regan in "Rapid Objective Refraction Using Evoked Brain Potentials", Investigative Opthamology, Vol. 12, No. 9, September 1973. In the Regan system, a light projector 20 projects light through a vibrating bar or check transparency 21, through a movable projection focus lens 22 onto a reflecting screen 23 which reflects the alternating stimulus back to the patient 24 through cylindrical lens 28. The evoked potentials are amplified by EEG amplifier 25 and applied to a filtering unit 26 which drives an XY plotter 27. The filter 26 is a synchronous filter that uses the properties of the pythagorean theorem to filter the input signal and produce an amplitude of the evoked potentials. Regan suggested that a digital Fourier transform could be substituted for the filter 26. A cathode ray tube or television has been suggested by Gomer in "Evoked Potential Correlates of Display Image Quality", Human Factors, October 1978, as suitable for an evoked potential stimulus source. Tyler et al in "Rapid Assessment of Visual Function: An Electronic Sweep Technique for the Pattern Visual Evoked Potential" uses a synchronous filter and integrator as a substitute for Regan's filter unit 26 and also used a sine wave grating with a reversal rate of 24 hertz to produce an XY plot of evoked potential amplitude versus spatial frequency for each discrete lens. Such a synchronous filter system requires at least 20 seconds for each sweep of each lens as acuity is being determined.